1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling an inverter which drives an induction motor at a variable speed, and in particular, to an automatic torque boost control method and an automatic torque boost control apparatus.
2. Description of the Related Art
In a known method to control an inverter to drive an induction motor at a variable speed, an output voltage V1 of the inverter is controlled in proportion to a primary frequency f1 of the inverter, i.e., V/f is kept fixed. The exciting current of an induction machine can be kept almost fixed in this method. However, when a load imposed onto the motor is increased, the voltage drop due to primary resistance 1 of the motor becomes greater and an induction or induced voltage of the motor is minimized. Resultantly, the exciting current of the motor is reduced and torque of the motor is lowered. Since an actual speed is lowered in response to a speed instruction, the speed regulation is increased. To overcome these difficulties, there have been proposed many automatic torque boost control methods to increase the motor torque particularly in a range of low and intermediate frequencies.
In general, a two-phase motor current is detected, the current is transformed from a fixed coordinate system to a rotating coordinate system (uvw/dq conversion) using a magnetic flux phase to obtain a torque current It. A voltage drop of primary resistance r1.multidot.It corresponding to the torque current It is added to fixed voltage (induction voltage) V/f. As a result, a voltage instruction Vm is produced as represented by expression (1). EQU Vm=K.multidot.f1*+r1.multidot.It (1)
where, * indicates an instruction value and K is a proportional gain.
A slip frequency fs proportional to the torque current It is added to a reference frequency fn* to obtain an inverter output frequency instruction f1* as follows. EQU f1*=fn*+fs=fn*+Ks.multidot.It (2)
where, Ks is a proportional gain.
In a method to detect a torque current from the two-phase current of the motor, instantaneous motor currents of two phases are transformed by a uvw/dq conversion, namely, from a fixed coordinated system to a rotating coordinate system. Therefore, the current of the torque can be instantaneously and correctly detected. The torque boost is automatically controlled according to the torque current. Consequently, the automatic torque boosting operation can be controlled with high response and high precision. However, this method requires motor current sensors or detectors for two phases and accordingly current sensor circuits for two phases. This soars the price of the inverter system.
JP-A-10-225199 describes a method to estimate a current vector using a motor current of one phase. In this method, alternating currents of other phases are estimated according to a state equation expressed by an inverter output voltage instruction, a motor current, and a motor impedance. The alternating currents are then transformed by a uvw/dq conversion to estimate current vectors. However, the state equation includes parameters, which in the motor control operation include such motor constants as a primary resistance, a secondary resistance, and mutual inductance. For each type of the motor, it is necessary to set these constants. This invention is related to relatively small-sized motors, which are produced in many types. For each type of the motor, it is disadvantageously required to collect the motor constants. The motor constants vary depending on a temperature corresponding to a load change. For example, the resistance value varies in a range from about 20% to about 30% of the original value according to an operating state. Therefore, a large error appears with respect to a setting value and the estimated current value is decreased in precision.